What Does 100-Year Flood Really Mean?

by Edwin Friedrichs, PE, PTP

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We often hear flood events described as a “1% Annual Chance Flood” or a “100-Year Flood.” These are actually the same event just expressed in different terms. The term “1% Annual Chance Flood” event is described in terms of Annual Exceedance Probability while the term “100-Year Flood” is described in terms of Average Recurrence Interval.

The descriptions “100-Year Flood” and “100-Year Flood Elevation” are two different technical terms related to precipitation events that often get confused. One is related to rainfall intensity over a certain period of time and the other one is the elevation of floodwater in a particular stream or water body associated with the event.

1% Annual Chance Flood or 100-Year Flood
These terms mean that a given rainfall intensity has a 1% chance of being equaled or exceeded in any one year at a specific location. It does not mean that flood water levels will reach the 1% Chance Flood Elevation.

1% Chance Flood Elevation or 100 Year Flood Elevation
These mean the water surface elevation above sea level that floodwater has been calculated to have a 1% chance of reaching in any given year at a specific location.

The 1% chance flood elevation is typically determined by utilizing sophisticated computer software and applying the 1% chance precipitation evenly across the entire watershed of a stream for a given time period—usually 24 hours.

100-Year Flood

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